1. Field of the Invention
The present invention relates generally to an apparatus for holding a semiconductor wafer, and more particularly for holding the edge of the semiconductor wafer in a desirable fashion when rotating the chuck and wafer arrangement at high speeds.
2. Description of the Related Art
Mechanisms used to examine the surface of a semiconductor wafer is commonly known as a handling chuck. Modern examination techniques entail incrementally inspecting the surface of the wafer while the wafer rotates, much like a phonograph needle passes over a phonograph record. As may be appreciated, the greater the speed of rotation, the larger the number of sample wafers which may be examined for a given time.
Prior systems have been employed to handle wafer during the examination process, but these systems have presented various undesirable limitations. For example, systems have been employed which provide a stable surface only below a particular revolution rate, such as 400 rpm. Further, previous designs have used different handling methods which may either grasp the rotating wafer in a way which could potentially damage the wafer or cause excessive motion when the wafer is rotated at high speeds. Excessive motion in either the lateral or axial directions during rotation of the wafer specimen translates into an inability to discern potential faults on the wafer surface. It has not been unusual for movement of the surface to be on the order of tens of microns in the axial direction, and on the order of 100 Hz. Either of these conditions severely diminishes the ability of an inspection system to discover actual defects.
An example of such a known edge handling wafer chucks is one which simply binds the wafer at three or four points and to hold the wafer at these fixed points during rotation. The problem with this design is that the vibration and contamination at excessive rpms causes damage to the wafer and inhibits accurate inspection due to excessive resonance.
Previous systems have also used vacuum chucks, which fixedly bind the wafer to a rotating surface using an air current drawn through the chuck to create a vacuum, thereby virtually fastening the wafer to the surface. The problem with such an arrangement is that during rotation the vacuum pressure may build up to an excessive level, creating prohibitively high forces between the wafer and the chuck surface due to the spinning vortex effects. This vacuum chuck arrangement also has the ability to cause the graininess of the silicon or other wafer components to break down due to the extensive contact between the wafer and the rotating surface.
For current wafer inspection systems it is desirable to maintain a stable surface for rotation speeds in excess of 400 rpm and up to and greater than approximately 1500 rpm. It is further desirable for surface vibration to be on the order of less than 1 micrometer under the aforementioned rotation speeds, with vibration frequency on the order of 100 Hz.
The design of such a system may entail various known materials, including a known plastic such as ertalyte. While it is known that the process of simply placing a wafer on a surface formed of a known plastic such as ertalyte causes the loss of thousands of particles, it is desirable in the spinning chuck environment to minimize the particle loss resulting from exposure of the wafer to the chuck and the rotation of the wafer at the aforementioned speeds.
It is therefore desirable to have an edge handling chuck which has the ability to operate in excess of 1500 RPM, while accurately centering and holding the wafer thereon, affording such features as minimal wafer surface contact, low turbulence resulting from contaminants such as ertalyte particles, and relatively low flutter.
It is therefore an object of the current invention to provide an apparatus for handling a silicon wafer during rotation thereof which permits securing the edge of the wafer without damaging the wafer.
It is a further object of the current invention to provide an apparatus for handling a silicon wafer during the rotation thereof that can operate in the range of and in excess of 1500 rpm while having the ability to both center the semiconductor wafer to within 0.5 millimeters and locate the wafer plane on the order of 12 micrometers.
It is another object of the current invention to provide an apparatus for handling a silicon wafer during the rotation thereof which minimizes particle loss resulting from exposure of the wafer to the apparatus.
It is still another object of the current invention to provide an apparatus for handling a silicon wafer during the rotation thereof which affords minimum surface contact between the apparatus and the wafer and a relatively constant force on the wafer edge.
It is still another object of the current invention to provide an apparatus for handling a silicon wafer during the rotation thereof which has low flutter when subjected to excessive rotation speeds.
The present invention is an edge handling chuck which operates to maintain a semiconductor wafer at a desirable orientation while rotating the wafer at high speeds. The edge handling chuck consists of a cylindrical plate which holds a silicon wafer using multiple spring loaded edge wafer clamps. The edge handling chuck is integrally supported on a shaft which is rotated using a spindle motor. The edge handling chuck and shaft are fixedly mounted to one another using a fixed plate. The shaft is hollow, thereby permitting passage of gas, such as air, through a center hole in the cylindrical plate. Gas passes through the center hole and is dispersed to the atmosphere using eight pressure relief openings in the cylindrical plate. The purpose of this gas arrangement is to stabilize the wafer due to spinning vortex effects. The anticipated size of the wafer is 300 millimeters, and the diameter of the cylindrical plate is approximately 13.5 inches.
The cylindrical plate has mounted therein an ertalyte ring located outside the pressure relief openings. This ertalyte ring provides an area of angled contact for the wafer. Various numbers of spring loaded edge wafer clamps may be employed, with three such clamps or eight such clamps, for example, used to hold the wafer.
The gas arrangement for the edge handling chuck operates to pass gas to the slight space between the semiconductor wafer and the cylindrical plate, thus into contact with the lower surface of the semiconductor wafer, and subsequently out of the arrangement using the pressure relief holes. These pressure relief holes provide the system the opportunity to expel gas received from the center hole out to atmosphere. The gas system thus provides an air bearing above the surface of the cylindrical plate and below the wafer lower surface. The gas pressure applied is chosen to provide sufficient force to flatten the wafer and counter the top pressure caused by the spinning vortex.
The system further includes a support plate for the purpose of raising and lowering the wafer clamps. The system moves the support plate by applying force using force application member to exert force on a centering device which thereupon pushes the wafer clamp cams, or arms, upward in order to open the wafer clamps.
The edge handling chuck therefore includes a rotatable surface with multiple fasteners, such as clamps, affixed to the rotatable surface. The fasteners are oriented to receive and relatively fixedly hold a specimen, such as a semiconductor wafer, in a desired orientation with respect to said rotatable surface during rotation thereof. The edge handling chuck also includes an extender/retractor system for engaging the plurality of fasteners to receive the specimen and affix the specimen proximate the rotatable surface. The system also has a gas bearing system, wherein the gas bearing system provides gas through the rotatable surface and into a space formed between the rotatable surface and specimen during the rotation thereof, and the gas bearing system comprises a pressure relief system, which is made up of multiple holes in the rotatable surface.
These and other objects and advantages of all of the aspects of the present invention will become apparent to those skilled in the art after having read the following detailed disclosure of the preferred embodiments illustrated in the following drawings.